The goal of this project is an understanding, in molecular terms, of the processes governing the differentiation of heterocysts, cells specialized for nitrogen fixation, along filaments of the cyanobacterium Anabaena. Although Anabaena is a prokaryote, heterocyst differentiation is characterized by irreversible commitment. The process includes changes in mRNA abundance, vast changes in the synthesis of specific protein, and several cases of rearrangement of DNA. Transcriptional regulation of the differentiation will be studied in vivo and in vitro, using cloned genes as probes of transcription. Many promoter regions have been identified; some of these will be modified by in vitro mutagenesis, returned to Anabaena by conjugation from E. coli, and analyzed on the basis of the activity of reporter genes fused to the modified promoter. Regulation of the process by which DNA around the genes encoding nitrogen fixation functions (nif) is rearranged will be studied by purification of the enzyme(s) responsible for site-specific recombination during heterocyst differentiation in the determination of factors that govern that enzyme's synthesis and activity.